Heavy hydrocarbons, e.g. bitumen, represent a huge natural source of the world's total potential reserves of oil. Present estimates place the quantity of heavy hydrocarbon reserves at several trillion barrels, more than 5 times the known amount of the conventional, i.e. non-heavy, hydrocarbon reserves. This is partly because heavy hydrocarbons are generally difficult to recover by conventional recovery processes and thus have not been exploited to the same extent as non-heavy hydrocarbons. Heavy hydrocarbons possess very high viscosities and low API (America) Petroleum Institute) gravities which makes them difficult, if not impossible, to pump in their native state. Additionally heavy hydrocarbons are characterised by high levels of unwanted compounds such as asphaltenes, trace metals and sulphur that need to be processed appropriately during recovery and/or refining.
Heavy hydrocarbon mixtures are challenging to transport from wells to refineries because they have very high viscosities making them difficult, and in some cases impossible, to pump. Pumping of high viscosity fluids is also expensive. Generally therefore the flowability of heavy hydrocarbon mixtures obtained from heavy hydrocarbon reservoirs needs to be improved through partial or full upgrading before transportation by pipeline or ship to a conventional refinery.
The transportability of viscous heavy hydrocarbon mixtures is conventionally improved by dilution with a lighter hydrocarbon such as naphtha, a very light crude oil or a condensate. The dilution of the heavy hydrocarbon with the diluent typically reduces its overall to API to about 20 degrees enabling it to be pumped to a refinery.
There are, however, disadvantages to the use of a diluent in this way. These include:                The need to transport diluent on-site. This problem becomes particularly acute for off-shore well sites.        The need to identify a compatible diluent for each heavy hydrocarbon mixture, e.g. one that does not cause precipitation of asphaltenes        The need to separate the diluent and the hydrocarbon mixture at the refinery prior to processing        Down stream processing/cleaning of the diluent prior to its reuse or disposal. Often it is preferable to return it to the well site (i.e. to recycle it) although this again requires it to be pumped a significant distance.        
Another approach that has previously been adopted is to upgrade heavy hydrocarbon mixtures on site prior to transportation to a refinery. Thus a heavy hydrocarbon mixture recovered from a well may be upgraded to form lighter oil having an API of about 20-35 degrees on site and then pumped to a refinery. In such a set up, the upgrading is typically carried out by thermal cracking and/or hydrocracking.
Again, however, there are disadvantages to such a process. These include:                The need to transport significant amounts of fuel and/or hydrogen for use in the upgrading processes to the well site.        The high level of contaminants in heavy hydrocarbon oils leads to catalytic poisoning as well as to the production of environmental pollutants.        
Moreover in order for full upgrading to be carried out economically on site, vast volumes of heavy hydrocarbon mixture need to be processed daily in order that the economies of scale make it feasible. Few well sites, however, produce sufficiently high volumes of heavy hydrocarbon mixture. This problem does not arise at conventional refineries that receive heavy hydrocarbon mixture from a number of different well sites.
Alternatively, a recovered heavy hydrocarbon mixture may be partially refined or upgraded on-site, e.g. using a processing plant located close to the production well. WO2005/003258, for example, discloses a process wherein part of a bitumen feed is upgraded and used to convert the overall feed into a pipeline-transportable crude oil. The process involves the following steps:
1. Separation of a bitumen feed into two parts, a first part and a second part.
2. Separation of the first part into light and heavy fractions, preferably by distillation.
3. Thermally cracking, e.g. by visbreaker soaking, the heavy fraction into a second light fraction and a residual fraction and fractionating said fractions.
4. Mixing the second part and the two light fractions to form a transportable hydrocarbon.
5. Using the residual fraction from thermal cracking for energy generation.
The process of WO2005/003258 is therefore relatively complex involving several energy intensive steps, e.g. distillation, thermal cracking and fractionation. This is undesirable, especially in a relatively remote location, e.g. offshore. Moreover a residual fraction of the bitumen feed is not incorporated into the pipeline-transportable crude oil and thus represents a loss in process yield.
A similar process is also disclosed in US2007/0108098. In this process a heavy hydrocarbon feedstock is separated into a residue component and a lighter component, e.g. by distillation, and then the lighter component is treated to produce a synthetic diluent that is combined with the residue component to make it transportable. Various different methods are disclosed for treating the lighter component including hydrocracking, hydrotreating, thermal conversion and catalytic cracking.
Like the process of WO2005/003258, however, the method of US2007/0108098 is quite complex and involves an initial separation step, such as a distillation, to produce a lighter fraction from which a diluent is subsequently produced. US2007/0108098 also suggests that the diluent may not necessarily be added to all of the residue component and that instead some of the latter may be diverted for use as a fuel. Again this represents a reduction in the process yield.
WO98/10036 describes an alternative process wherein a part of a heavy oil is separated out and is degraded to a more liquid substance which is then mixed with the remaining heavy oil. The process described in WO98/10036 does not involve an initial separation of the heavy hydrocarbon mixture into lighter and heavier fractions as in WO2005/003258 and US2007/0108098, but it does involve modifying the composition of the part of the heavy oil that is to be upgraded. Thus in the method disclosed in WO98/10036 the separated heavy oil is mixed with solid particles, typically sand, and the mixture upgraded by cracking in a hammer mill type of apparatus. Water is also preferably added prior to carrying out the cracking. In the cracking process the heat required is supplied at least partially by the effect of the hammers of the mill (i.e. mechanically). After the cracking reaction is complete, the crude reaction mixture undergoes a separation process to remove as much of the solid particles and water as possible before the cracked, lighter hydrocarbon is mixed with the remaining heavy oil.
The process of WO98/10036 therefore requires a significant number of steps in addition to the actual cracking process to produce an upgraded hydrocarbon for mixing with the remaining heavy oil. Methods and equipment are required for adding solid particles to the heavy oil, e.g. in the cracker apparatus, as well as for removing them from the upgraded product. Additionally to make the process economically more attractive, the solid particles, e.g. sand, needs to be cleaned to remove the hydrocarbon stuck to its surface so it can be recycled in the process. WO98/10036 also teaches that the presence of water stabilises the hammer mill cracking process and hence water should be added to the separated part of the heavy oil prior to processing. The use of 1-20% by weight water is advocated. This increases the volume of material that undergoes cracking and hence the energy consumed in the cracking process. Whilst this is clearly worthwhile if it means a stable hydrocarbon product may be obtained, it is clearly not ideal from a cost point of view.
U.S. Pat. No. 5,069,775 discloses another alternative process wherein about 50% of a crude heavy oil recovered from a formation is directed to a reactor wherein it undergoes hydroconversion to produce a low viscosity fraction. After hydroconversion is complete, the lower viscosity product passes through two separators in sequence and the bottoms therefrom is mixed with the remaining 50% of heavy crude that bypasses the upgrading section. The resulting product is described as a flowable crude that can be pumped through a pipeline.
Like WO98/10036 the process disclosed in U.S. Pat. No. 5,069,775 does not involve an initial separation of the heavy hydrocarbon mixture into fractions as in WO2005/003258 and US2007/0108098 discussed above. The method, however, requires the hydroconverted product to undergo two separation processes which each remove hydrogen as well as lighter hydrocarbons. This is disadvantageous. It means, for example, that the lightest hydrocarbons having the lowest viscosity are not incorporated into the upgraded hydrocarbon mixture and that not all of the recovered hydrocarbon is present in the upgraded hydrocarbon mixture, i.e. it represents a reduction in process yield. The need for separation of the hydroconverted product also increases the number of steps involved in the process and introduces the need for separation equipment.
A need therefore exists for alternative processes for treating heavy hydrocarbon mixtures to improve their transportability. Simple and economically attractive processes are clearly desirable.